Some footprint always have a shift

[villannm]

Hi !
After doing a lot of tests, i saw that the position of some footprint always have an offset (by exemple here, the SOT23-5 footprint and a perfectly placed SOT on the same board)

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Do you know where this offset could come from ?

[Stefan]

Is the peice of paper with the image of the circuit board completely flat? I was running the Hello World job last night to get used to the software and found that since one corner of the paper was slightly curled up, the fiducial in that area was in a slightly wrong X location. This in turn impacts where the software decides non-fiducial features are located.

[JuKu]

How much is the error? SOT23-5 is rather small; you might be seeing:
-Imperfection of your printer and/or paper. As noted, it doesn’t take much wrinkle to get the measurement slightly off.
-Error in the footprint itself. Unlikely, but you wouldn’t be the first to find a mistake in your part library.
-And finally, you might be seeing what the machine can do. If the part is actually placed where the mark shows, it would be quite ok. In the backslash thread page 3 is some discussion about the limits of the machine. Here is a clip:

[Stefan] ... I had to micro jog the machine 0.02-0.03mm back

This is very close to what the current design can do even in theory: The pulley and belt give 40mm movement on one rotation. 0.9 deg/step is 400 steps per rev, so one step is 0.1mm With 8x microstepping, you in theory get 0.0125mm resolution. But microstepping is not 100% accurate. In our case, there are no forces pushing on the gantry, unlike on CNC machines. Still, we can expect that microstepping only gets close to the theoretical point. The 0.02 -0.03mm is about +- 1 microstep. (FYI, I'm currently experimenting with 4x microsteps. The machine moves faster, and I don't see much difference in accuracy.)

[villannm]

Hi !
Thanks for your answers ^^
The paper is not perfectly flat, but on the camera it make no differences on the placements of the fiducials and the components locations are on a flat part. And only some kinds of components have the shift, so i dont think thats the reason...

How can i determine if my footprints have a mistake ? I looked at it and the center of the footprint is exactly the center of the component.
We want to try to place FQN 144 component, with only a 0,25mm pitch, that's why i want to be as precise as possible. But if this kinf of components are too hard to place automatically, we may use the manual placing.

[JuKu]

How can i determine if my footprints have a mistake ? I looked at it and the center of the footprint is exactly the center of the component.

That's how.

Almost always, the component is correct, but I've seen cases where the center is calculated from pads (and fooled by pads only one side of the component, mounting tabs or heatsink copper) and a case where the center was from all graphics on the cad part, not from copper and outline only.

Off-topic, but you do want to keep an eye on parts where there are pins only on one side of the component and check, that the center you have on your cad system matches the center that the part comes on a tape.